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  <meta name="description" content="使用@Import可以导入一个指定的@Configuration类作为一个bean（@Configuration类中定义的bean自然会注入），也可以使用@ImportResource导入other non-{@code @Configuration} bean definition resources；或者使用ImportSelector或ImportBeanDefinitionRegistra">
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          Spring Boot @Import的使用
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        <p>使用@Import可以导入一个指定的@Configuration类作为一个bean（@Configuration类中定义的bean自然会注入），也可以使用@ImportResource导入other non-{@code @Configuration} bean definition resources；或者使用ImportSelector或ImportBeanDefinitionRegistrar来选择加载的bean。@Import导入的bean在所在类上使用@Autowired就可注入</p>
<p> 比如，@SpringBootApplication类默认只导入所在包层级及以下层级的bean，在这一个包之外的@Configuration类就需要使用@Import才能注入（这种情况可以使用@ComponentScan等）；比如，使用ImportSelector或ImportBeanDefinitionRegistrar，配合自定义注解，根据某些参数或者配置加载bean</p>
<p> 如自定义starter时使用的@EnableAutoConfiguration，使用了@Import(AutoConfigurationImportSelector.class)注解，AutoConfigurationImportSelector类中从META-INF&#x2F;spring.factories找出所有需要加载的configurations返回</p>
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<h2 id="使用-Import与ImportSelector的自定义-EnableXXX"><a href="#使用-Import与ImportSelector的自定义-EnableXXX" class="headerlink" title="使用@Import与ImportSelector的自定义@EnableXXX"></a>使用@Import与ImportSelector的自定义@EnableXXX</h2><p>假设某个数据源存在A、B两种连接方式，希望可以根据指定的参数选择使用的方式</p>
<p>连接方式相关的配置类简化如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// Conn类，代表一个数据源连接</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Conn</span> &#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> String connName;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">public</span> <span class="title function_">Conn</span><span class="params">(String connName)</span> &#123;</span><br><span class="line">        <span class="built_in">this</span>.connName = connName;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">public</span> String <span class="title function_">getConnName</span><span class="params">()</span> &#123;</span><br><span class="line">        <span class="keyword">return</span> connName;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">setConnName</span><span class="params">(String connName)</span> &#123;</span><br><span class="line">        <span class="built_in">this</span>.connName = connName;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 连接方式A的配置类</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">ConfigurationA</span> &#123;</span><br><span class="line">    <span class="meta">@Bean</span></span><br><span class="line">    <span class="keyword">public</span> Conn <span class="title function_">conn</span><span class="params">()</span> &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">Conn</span>(<span class="string">&quot;A&quot;</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 连接方式B的配置类</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">ConfigurationB</span> &#123;</span><br><span class="line">    <span class="meta">@Bean</span></span><br><span class="line">    <span class="keyword">public</span> Conn <span class="title function_">conn</span><span class="params">()</span> &#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">Conn</span>(<span class="string">&quot;B&quot;</span>);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br></pre></td></tr></table></figure>

<p>提供一个注解@EnableConn，用于导入Conn bean，并在注解上指定@Import(ConnImportSelector.class)来选择使用A或者B：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// @EnableConn，导入Conn bean，并指定使用ConnImportSelector来选择加载A或者B</span></span><br><span class="line"><span class="meta">@Target(ElementType.TYPE)</span></span><br><span class="line"><span class="meta">@Retention(RetentionPolicy.RUNTIME)</span></span><br><span class="line"><span class="meta">@Inherited</span></span><br><span class="line"><span class="meta">@Import(ConnImportSelector.class)</span></span><br><span class="line"><span class="keyword">public</span> <span class="meta">@interface</span> EnableConn &#123;</span><br><span class="line"></span><br><span class="line">    ConnMode <span class="title function_">useConn</span><span class="params">()</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// ConnImportSelector，根据注解传入的参数选择加载的Conn</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">ConnImportSelector</span> <span class="keyword">implements</span> <span class="title class_">ImportSelector</span> &#123;</span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="keyword">public</span> String[] selectImports(AnnotationMetadata annotationMetadata) &#123;</span><br><span class="line"></span><br><span class="line">        <span class="type">AnnotationAttributes</span> <span class="variable">annotationAttributes</span> <span class="operator">=</span></span><br><span class="line">                AnnotationAttributes.fromMap(annotationMetadata.getAnnotationAttributes(EnableConn.class.getName()));</span><br><span class="line"></span><br><span class="line">        <span class="comment">// ConnMode是自定义的代表A、B两种连接方式的枚举</span></span><br><span class="line">        <span class="type">ConnMode</span> <span class="variable">conn</span> <span class="operator">=</span> annotationAttributes.getEnum(<span class="string">&quot;useConn&quot;</span>);</span><br><span class="line">        <span class="keyword">switch</span> (conn) &#123;</span><br><span class="line">            <span class="keyword">case</span> A:</span><br><span class="line">                <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">String</span>[]&#123;ConfigurationA.class.getName()&#125;;</span><br><span class="line">            <span class="keyword">case</span> B:</span><br><span class="line">                <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">String</span>[]&#123;ConfigurationB.class.getName()&#125;;</span><br><span class="line">            <span class="keyword">default</span>:</span><br><span class="line">                <span class="keyword">return</span> <span class="literal">null</span>;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">enum</span> <span class="title class_">ConnMode</span> &#123;</span><br><span class="line">    A,</span><br><span class="line">    B</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>这样，在@SpringBootApplication类上添加@EnableConn注解并指定useConn参数，指定使用的bean：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@EnableConn(useConn = ConnMode.B)</span></span><br><span class="line"><span class="meta">@SpringBootApplication</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">ImportTestApplication</span> &#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">main</span><span class="params">(String[] args)</span> &#123;</span><br><span class="line">        SpringApplication.run(ImportTestApplication.class, args);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// Controller中注入的Conn是@EnableConn(useConn = ConnMode.B)指定的B方式的实现</span></span><br><span class="line"><span class="meta">@RestController</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">TestController</span> &#123;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Autowired</span></span><br><span class="line">    <span class="keyword">private</span> Conn conn;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@GetMapping(&quot;/test&quot;)</span></span><br><span class="line">    <span class="keyword">public</span> String <span class="title function_">test</span><span class="params">()</span> &#123;</span><br><span class="line">        <span class="keyword">return</span> conn.getConnName();</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br><span class="line"></span><br></pre></td></tr></table></figure>

<p>使用ImportBeanDefinitionRegistrar接口的方式类似，但是可以自行构建BeanDefination添加到传入的BeanDefinitionRegistry中</p>
<p>以上是一个简单的例子，实际上在selectImports()方法传入的参数annotationMetadata中，可以得到自定义注解所在类的其他注解的信息、类的信息，这样可以实现其他功能，比如可以实现自定义扫描路径加载bean等等</p>

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